Most modern vehicles have power steering in which the force exerted by the operator on the steering wheel is assisted by hydraulic pressure from an electric or engine-driven pump. The force applied to the steering wheel is multiplied by the mechanical advantage of a steering gear. In many vehicles, the steering gear is a rack and pinion, while in others it is a recirculating ball type.
Electric power steering has started to replace hydraulic power steering in some vehicles for fuel economy. One way this is accomplished is through the reduction or elimination of losses inherent in traditional steering systems. Therefore, electric power steering typically requires power only on demand. Commonly, in such systems an electronic controller is configured to require significantly less power under a small or no steering input condition. This dramatic decrease from conventional steering assist is the basis of the power and fuel savings. Electric power steering has several additional advantages. The steering feel provided to the operator has greater flexibility and adaptability. Overall system mass savings may also be achieved. Electric power steering is powerplant independent, which means it can operate during an all-electric mode on a vehicle.
Furthermore, polyphase permanent magnet (PM) brushless motors excited with a sinusoidal field provide lower torque ripple, noise, and vibration when compared with those excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. Such deviations usually result in parasitic torque ripple components at various frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.
One method of torque control for a permanent magnet motor with a sinusoidal, or trapezoidal back EMF is accomplished by directly controlling the motor phase currents. This control method is known as current mode control. The phase currents are actively measured from the motor phases and compared to a desired profile. The voltage across the motor phases is controlled to minimize the error between the desired and measured phase current. However the current mode control require multiple current sensors and A/D channels to digitize the feedback from current sensors, which would be placed on the motor phases for phase current measurements.
Another method of torque control is termed voltage mode control. In voltage mode control, the motor phase voltages are controlled in such a manner as to maintain the motor flux sinusoidal and motor back emf rather than current feedback is employed. The voltage mode control does not require precise current measurement form multiple current sensors. One application for an electric machine using voltage mode control is the electric power steering system (EPS).
In voltage mode control the amplitude and phase angle of phase current vector is calculated based on the motor back emf, position and motor parameters (e.g., resistance, inductance and back emf constant). A sinusoidal instantaneous line voltage based on the calculated phase and amplitude vector of phase voltage is applied across the motor phases. An instantaneous value of voltage is realized across the phases by applying a pulse width modulated (PWM) voltage the average of which is equal to the desired instantaneous voltage applied at that position of the motor.
There are different methods of profiling the phase voltages in order a achieve a sinusoidal line to line voltage and therefore the phase current in a wye-connected motor. A conventional approach is to apply sinusoidal voltages at the phase terminals. In this method the reference for the applied voltage is at half the dc bus voltage (Vdc/2). In another approach, the phase voltage is referenced to the power supply ground (instead of Vdc/2 as in conventional way). This is achieved by applied a zero voltage for 120 electrical degrees at each phase terminal during one electric cycle. This method increases the voltage resolution while reducing the switching losses.
EPS control systems employing voltage mode control algorithm, uses the amplitude and phase angle of the voltage for torque control. In order to produce the accurate torque from the motor it is important to apply and control both amplitude of the voltage and its phase angle as accurately as possible. Although the voltage mode control does not require the motor current for torque control, the current torque and flux components of motor current are desired to observe the motor parameter changes during the operation and life and diagnostics purposes.